1. Field of the Invention
The present invention relates to an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu having a high critical current density, and a method for manufacturing the same.
2. Description of the Related Art
An oxide substrate or an oxide board such as MgO, alumina or YSZ, or a metal substrate or a metal board such as Ag, Au, Pt or Ni is formed with an oxide superconductor in film form to function, so as to devise a variety of applied products.
As a method of forming this oxide superconductor into film form, a technique is tried in which oxide superconductive synthetic powder that is oxide superconductor powder, to which an appropriate organic binder is added, is made into paste form, thereafter applied to the surface of the substrate or the board using the screen printing method, the doctor blade method, the spray method, or the like, and burned to thereby form a polycrystalline oxide superconductor thick film.
This technique of forming the oxide superconductor thick film has a quite low manufacturing cost, that is, an advantage because of no need for an expensive single crystal substrate and a large-scale and expensive apparatus requiring a high vacuum system represented by PVD, CVD or the like, and thus it is considered as the technique closest to practical use.
Conventionally, as an oxide superconductor material for forming this thick film, it has mainly been tired to use the YBCO family having a composition of Y1Ba2Cu3Ox, the Bi2212 family having a composition of Bi2Sr2Ca1Cu2Oy, and the Bi2223 family having a composition of (Bi, Pb)2+nSr2Ca2Cu3Oz. (Note that, generally, 0<a<0.5, and this applies to the following.)
When the oxide superconductor thick film is applied here to a practical product, a critical current density (hereafter, described as “Jc”) of 3,500 A/cm2 or more is required at the temperature of liquid nitrogen.
From this point of view, the conventional art of the above-described oxide superconductor is briefly explained.
For example, an oxide superconductor thick film (hereinafter, described as a “thick film”) containing Y1Ba2Cu3Ox can attain a Jc of several hundred A/cm2 at the temperature of liquid nitrogen. A critical current value (hereafter, described as “Ic”) of about several A can be attained in a thick film sample thereof having a width of 5 mm and a thickness of 100 μm.
It is difficult, however, to align the crystal orientation of Y1Ba2Cu3Ox in the form of the thick film of the oxide superconductor, that is, a polycrystal, and accordingly it is difficult to attain the aforesaid Jc or more.
Further, for example, a thick film containing Bi2Sr2Ca1Cu2Oy can attain a Jc of 6000 A/cm2 at the temperature of liquid nitrogen, an Ic of about 10 A to about 15 A in a thick film sample thereof having a width of 5 mm, a thickness of 50 μm and so on, and thus it is a potential material.
In this material, however, superconducting characteristics sharply decrease as the film thickness of the sample is increased to attain a higher Ic. As a result, with an increase in film thickness, the value of Jc, obtained by averaging the values of the whole thick film, decreases, such that even a thick film produced having, for example, a width of 5 mm and a thickness of 100 μm has an Ic of about 20 A and a Jc of as small as about 4000 A/cm2. Even if the film thickness is increased more, the Ic does not increase in proportion thereto, but conversely the averaged Jc keeps on decreasing.
Further, for example, as for a thick film containing (Bi, Pb)2+aSr2Ca2Cu3Oz, a thick film sample having a thickness of 100 μm is currently manufactured and can attain a Jc of several hundred A/cm2 at the temperature of liquid nitrogen, and an Ic of about several A in a thick film sample thereof having a width of 5 mm. The Jc exhibited by this thick film, however, is about one-tenth of the value expected from the value of a Jc offered by a similar bulk body containing (Bi, Pb)2+aSr2Ca2Cu3Oz. Furthermore, when the film thickness of the thick film is increased to be 100 μm or more to attain a higher Ic, the oxide superconductor peels from the surface of the substrate or the board in the aforesaid burning, and thus the oxide superconductor containing (Bi, Pb)2+aSr2Ca2Cu3Oz is considered to be difficult to use as a thick film material.